1. Field of the Invention
This invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
2. Description of the Related Art
In general, a catalyst converter including a ternary catalyst which simultaneously purifies HC, CO and NOx in exhaust gas is disposed in the exhaust passage of an internal combustion engine. With the catalyst in the catalyst converter, purification rates heighten for all of the components HC, CO and NOx near a theoretical air-fuel ratio. Usually, therefore, an O2 sensor is disposed on the upstream side of the catalyst, and an air-fuel ratio is controlled on the basis of the detection value of the upstream-side O2 sensor so as to become near the theoretical air-fuel ratio.
The upstream-side O2 sensor which is disposed on the upstream side of the catalyst is located in that place of an exhaust system which is as close to the combustion chamber of the internal combustion engine as possible, that is, at the aggregate part of an exhaust manifold in the upstream of the catalyst. Accordingly, the upstream-side O2 sensor is exposed to a high exhaust temperature and is poisoned with various harmful substances, so that the output characteristic thereof fluctuates greatly. In order to compensate the fluctuation of the output characteristic, therefore, a double O2 sensor system has already been proposed. In the system, a downstream-side O2 sensor is disposed on the downstream side of the catalyst, and a second air-fuel ratio feedback control based on the downstream-side O2 sensor is performed in addition to a first air-fuel ratio feedback control based on the upstream-side O2 sensor (refer to, for example, Patent Document 1 being JP-A-63-195351, and Patent Document 2 being JP-A-6-42387).
In such a prior-art air-fuel ratio control apparatus for the internal combustion engine, the downstream-side O2 sensor is lower in response rate than the upstream-side O2 sensor, but it has merits as stated below. The downstream-side O2 sensor is little influenced by heat because an exhaust temperature is low on the downstream side of the catalyst, and it is little poisoned because the various harmful substances have been trapped by the catalyst, so that the fluctuation of the output characteristic of the downstream-side O2 sensor is small. Further, since the exhaust gas is mixed more on the downstream side of the catalyst, the purification state of the catalyst located upstream of the downstream-side O2 sensor can be detected stably.
Besides, in accordance with the prior-art air-fuel ratio control apparatus for the internal combustion engine as employs the double O2 sensor system, the air-fuel ratio of the upstream side with respect to the catalyst is corrected, and the output of the downstream-side O2 sensor is maintained at a target value, whereby the fluctuation of the output characteristic of the upstream-side O2 sensor can be compensated, and the purification state of the catalyst can be held favorable.
Further, the catalyst is endowed with an oxygen storage capability in order to absorb the temporary fluctuation of the upstream-side air-fuel ratio from the theoretical air-fuel ratio. That is, in a case where the air-fuel ratio is on a lean side with respect to the theoretical air-fuel ratio, the catalyst accepts and accumulates oxygen contained in the exhaust gas, and in a case where the air-fuel ratio is on a rich side, the catalyst emits the oxygen accumulated therein. In this manner, the catalyst acts like a filter or means for averaging the air-fuel ratio, and the fluctuation of the air-fuel ratio on the upstream side is averaged within the catalyst and becomes the air-fuel ratio of the downstream side of the catalyst.
Besides, the upper limit value of an oxygen storage quantity is determined by the quantity of a substance which has the oxygen storage capability and which is added at the manufacture of the catalyst. Accordingly, when the oxygen storage quantity has reached its upper limit value or its lower limit value “0”, the catalyst can no longer absorb the fluctuation of the upstream-side air-fuel ratio, the air-fuel ratio within the catalyst deviates from the theoretical air-fuel ratio, and the purification capability of the catalyst degrades. On this occasion, the air-fuel ratio of the downstream side deviates greatly from the theoretical air-fuel ratio, and hence, the saturation of the oxygen storage quantity to the upper limit value or the lower limit value “0” can be detected.
The purification rates of the catalyst for all the components HC, CO and NOx in the exhaust gas become high near the theoretical air-fuel ratio, and they become the highest in a case where the oxygen storage quantity of the catalyst is about half of the upper limit value thereof. Besides, the catalyst oxygen storage quantity intermediate between the upper limit value and the lower limit value can be detected on the basis of the minute change thereof near the theoretical air-fuel ratio of the air-fuel ratio on the downstream side. Therefore, the purification rates of the catalyst can be kept high in such a way that the oxygen storage quantity is controlled to about the half of the upper limit value by controlling the output of the downstream-side O2 sensor to the target value.
In general, during fuel cut for which fuel feed into the internal combustion engine is stopped, the air-fuel ratio of the upstream side becomes sharply lean. Accordingly, the oxygen storage quantity of the catalyst increases rapidly and arrives at the upper limit value, and the purification characteristic of the catalyst worsens drastically. Therefore, the purification characteristic of the catalyst needs to be recovered in such a way that, after the restart of the fuel feed, the oxygen storage quantity of the catalyst is reset to the appropriate quantity being about the half of the upper limit value, as quickly as possible.
Besides, since the catalyst in the catalyst converter is exposed under the exhaust gas temperature of the high temperature, it is designed so that its function may not abruptly degrade under service conditions which are usually considered for a vehicle. However, in a case where the exhaust gas temperature has become abnormally high for any cause, for example, misfire during the running of the internal combustion engine, the upper limit value of the oxygen storage quantity of the catalyst lowers conspicuously. Besides, even under the usual service conditions, when the travel distance of the vehicle reaches several tens-of-thousands kilometers, the upper limit value of the oxygen storage quantity lowers gradually due to the secular change of the catalyst. Accordingly, the lowering of the upper limit value of the oxygen storage quantity attributed to the deterioration of the catalyst correlates with the degradation of the exhaust gas purification performance of the catalyst, and the deterioration of the catalyst can be detected by detecting the lowering of the upper limit value of the oxygen storage quantity. When the deterioration of the catalyst proceeds, environmental pollution is incurred. It is therefore necessary to detect the deterioration of the catalyst exceeding an allowable range and to notify the deterioration to a user by an alarm lamp or the like, whereby the user is prompted to exchange the catalyst.
FIGS. 19A and 19B are characteristic diagrams each showing the change of the output V2 of the downstream-side O2 sensor, and FIG. 19A corresponds to a case where the catalyst is normal, while FIG. 19B corresponds to a case where the catalyst has been deteriorated. In the case of the deteriorated catalyst shown in FIG. 19B, as compared with the case of the normal catalyst shown in FIG. 19A, a time period in which the output V2 of the downstream-side O2 sensor is reset to the target value after the point of time t1 of the release of a fuel cut state for cutting the fuel feed becomes shorter as the upper limit value of the oxygen storage quantity of the catalyst decreases more due to the deterioration of the catalyst. The reason therefor is that the variation of oxygen storage required for being reset from the upper limit value to about the half of this upper limit value decreases with the decrease of the upper limit value of the oxygen storage quantity, so the resetting time period shortens with the same air-fuel ratio control. Therefore, an apparatus wherein the resetting time period in which the output V2 of the downstream-side O2 sensor is reset to the target value since the point of time t1 of the release of the fuel cut is measured, thereby to render the deterioration decision of the catalyst, has already been proposed (refer to, for example, Patent Document 3 being JP-A-2-33408 or Patent Document 4 being JP-A-2-136538).
The prior-art apparatus disclosed in Patent Document 3 or 4 utilizes the arrival of the catalyst oxygen storage quantity at the upper limit value by the fuel cut, and it has a full set of conditions before the start of a deterioration diagnosis and need not perform any special air-fuel ratio control such as leaning the upstream-side air-fuel ratio, before the start of the diagnosis. Besides, while NOx emission is apprehended to increase in case of performing the leaning, the apparatus has the merit that the increase of the NOx emission is not apprehended during the fuel cut. Further, the apparatus diagnoses the catalyst deterioration by utilizing the behavior that the output of the downstream-side O2 sensor is reset to the target value automatically by the second air-fuel ratio feedback control, and it need not perform any special air-fuel ratio control such as enriching the upstream-side air-fuel ratio during the diagnosis. Besides, when the enrichment is made, it is apprehended that the catalyst oxygen storage quantity will be saturated to the lower limit value, and that the emission of the components HC and CO will increase. Since, however, the second air-fuel ratio feedback control is utilized, the worsening of the exhaust gas is not incurred.
With such a prior-art apparatus, the precision of the deterioration decision of the catalyst is high in a case where the behavior of the second air-fuel ratio feedback control is the same every time, or under such an identical condition that an idling running continues after the release of the fuel cut. However, the apparatus has had the problem that the precision of the deterioration decision of the catalyst worsens sharply in a case where the gain of the second air-fuel ratio feedback control has changed, or in a case where a running condition has fluctuated in such a manner that the vehicle is accelerated or decelerated after the release of the fuel cut. The problem is ascribable to the fact that the behavior of the oxygen storage quantity of the catalyst cannot be precisely represented merely by a time period measurement.
Besides, the changing speed of the oxygen storage quantity in the catalyst is proportional to the deviation of the upstream-side air-fuel ratio from the theoretical air-fuel ratio and a suction air quantity. Therefore, in a case where the manipulation quantity of the upstream-side air-fuel ratio from the theoretical air-fuel ratio has been changed by the change of the gain of the second air-fuel ratio feedback control, the changing speed of the oxygen storage quantity changes, and hence, the resetting time period increases or decreases to degrade the deterioration decision precision. Further, in a case where the suction air quantity has been changed by the acceleration or deceleration, the changing speed of the oxygen storage quantity changes, and hence, the resetting time period increases or decreases to degrade the deterioration decision precision.
Besides, in a case where a λ (lamda) O2 sensor whose output changes abruptly near the theoretical air-fuel ratio is employed as the upstream-side O2 sensor, the deviation of the upstream-side air-fuel ratio from the theoretical air-fuel ratio cannot be detected on account of the two-valued characteristic of the λ O2 sensor. Therefore, the apparatus has had such a problem that the behavior of the oxygen storage quantity of the catalyst cannot be calculated by considering also the air-fuel ratio of the upstream side.